The present invention relates to the fabrication of integrated circuitry and, more particularly, to increasing the etch selectivity of sacrificial silicon dioxide or other material.
Ion implantation is commonly used in the fabrication of integrated circuitry to provide desired doping. Precise dopant distribution profile, dopant concentration and depth of implantation is attained by carefully choosing the parameters of implantation and those of the subsequent thermal annealing of the target. When ion implantation is used in the initial stages of integrated circuitry fabrication to provide doping for voltage threshold purposes, it is common to grow a silicon dioxide layer first to protect the substrate. This thermal oxide layer generally is referred to as a "sacrificial" oxide layer. (It also is sometimes referred to as the "first gate" oxide.) It typically is removed after ion implantation by subjecting the substrate to a diluent solution of hydrofluoric acid (HF) which may or may not be buffered.
It is desirable in some fabrication processes to control the etch rate of the sacrificial oxide relative to other material on the same substrate that may come into contact with the etchant. For example, additional silicon dioxide material sometimes is deposited or grown in certain regions of a substrate, e.g., for shallow trench isolation, prior to exposure of the substrate to an etchant to remove the sacrificial oxide. This other oxide is commonly referred to as a field oxide. It is important that this field oxide maintain its planarity in view of further processing, e.g., the formation of crossover conductors on the same. Field oxide loss due to etching of a sacrificial layer is even a bigger problem if the field oxide is a LPCVD (low pressure chemical vapor deposition) layer, since field oxide deposited in this manner etches roughly 20% faster than conventional thermal oxide when both are exposed to aqueous HF. Thus it is desirable to increase the etch rate of the sacrificial oxide so as to reduce unwanted etching of the field oxide.